Parkinson's disease (PD) culminates in dementia, immobility, and death at a huge societal cost. Even early in the course, motor and cognitive dysfunction impairs instrumental activities of daily living (IADL). Non-motor symptoms due to fatigue, mood, sleep, and autonomic disorders further reduce quality of life (QoL). DTI shows progressive decline in brain tissue integrity. Usual care of PD centers on medical and surgical treatments relieve motor symptoms, but these cause side effects and lose efficacy over time. Usual treatment for non- motor manifestations with pharmaceuticals (e.g., antidepressants) is symptomatic and not specific for PD. Acetylcholine esterase inhibitors exert modest symptomatic benefits on dementia, but there is no approved treatment for mild cognitive impairment. Physical Therapy is usually prescribed in later stages when mobility impairment ensues. There is no approved standard exercise regimen for PD. There is no cure or disease modifying treatment. Thus, there is a critical need for treatments that provide broad spectrum of benefits and slow PD. Preliminary research suggests that aerobic exercise has potential to meet this need. However, aerobic exercise is demanding and carries some risks. It is unknown if aerobic exercise is more beneficial than usual care in PD in long term due to gaps in our knowledge about the effects of cardiorespiratory fitness (CRF) on brain tissue integrity, motor function, cognition, IADL, QoL, and disease progression. Limitations of current studies include short duration, small sample size, lack or inadequacy of controls, lack of outcome measures for cognition and IADL, and lack of biological markers to measure progression. Our objective in this application is to fill the translational gap by determining the biological, clinical, and functional effects of long term aerobic exercise (LTAE) in PD. Our overall hypothesis is that LTAE improves brain tissue integrity and slows down PD. Our FIRST AIM is to determine the effects of LTAE on clinical features and functional abilities in PD. Our prior 6-month, uncontrolled trial showed preliminary evidence that aerobic exercise improves aspects of motor function, cognition, and QoL in PD, but long term outcomes and implication for functional abilities are unknown. We hypothesize that LTAE will provide sustained improvement in motor function, cognition, and non- motor symptoms with translation of benefits to QoL and IADL. We will test this with a one-year randomized controlled trial (RCT) that compares the effects of moderate aerobic exercise vs usual care. We will use driving as the outcome for IADL. Driving represents an important symbol for independence, and depends on integrity of cognitive and motor systems. Our SECOND AIM is to determine the mechanism of LTAE effects in PD. CRF reflects complex improvements in vascular, cardiac, and metabolic health from aerobic exercise. There is preliminary evidence that higher CRF is associated with better brain health and motor/cognitive function, and that aerobic exercise improves these outcomes. For example, our preliminary study showed improvement of microtissue integrity in the striatum and white matter on DTI, but it is unclear how these changes counteract PD progression over long term. Our hypotheses are: 1) LTAE will improve brain tissue integrity as indexed by DTI, 2) LTAE effects on motor and cognitive function are mediated by changes in brain tissue integrity on DTI, and 3) physiological processes leading to improved CRF from AE are critical to the benefits on the brain tissue integrity and motor/cognitive function. We will test these hypotheses determining the effects of LTAE on CRF and DTI, and the association between individual differences in training-related changes in motor and cognitive function, DTI, and CRF. In summary, our proposal leverages our diverse interdisciplinary team, strong preliminary data and past work, and unique infrastructure to determine if LTAE slows down neurodegeneration and clinical disability in PD.